Sending Mail: External (Internet) User to an Internal
User

Messages addressed from an external user
(from the Internet) to an internal user go to either MTA Inbound Relay 1 or
2 (a load balancer is not required). The inbound relay reads the address and
determines that the message is addressed to an internal user. The inbound
relay determines by using an LDAP lookup whether to send it to Message Store
1 or 2, and delivers accordingly. The appropriate Message Store receives the
message and delivers it to the appropriate mailbox.